1. Field of the Invention
This invention relates to a material for making a fuse which generates a small quantity of heat, which has a long useful service life and which is suitable for use in a circuit where a relatively large current flows.
2. Prior Art
Tubular or planar fuses made of zinc have been widely used hitherto in various circuits in automobiles. A demerit of the fuse made of zinc is its short useful service life although the fuse has such a merit that it generates a small quantity of heat before it breaks the circuit by melt-down and during use under a normal current-conducting condition. With a view to solve the above problem, a fuse made of a zinc-copper alloy containing less than 5% of copper added to zinc has been proposed, as disclosed in JP-A-No. 53-138918. However, it has been pointed out that, when such a fuse is connected in a circuit where a relatively large current of, for example, 20 amperes or more flows, especially when used in a circuit where an inrush current flows due to repeated on-off of a motor, the alloy particles tend to become excessively large due to repeated thermal expansion and contraction, resulting in development of cracks. Thus, the proposed fuse made of such a zinc-copper alloy has a limited useful service life.
Therefore, a fuse made of a copper alloy having a high melting point, as disclosed in JP-A-No. 58-163127, is now commonly used in a circuit where a large current flows. However, the fuse made of such a copper alloy is disadvantageous in that it generates a large quantity of heat although it has a long useful service life.
Generally, the circuit breaking action of a fuse made of a metal takes place when an area of the fuse heated by the joule heat melts down at the melting point of the metal, thereby breaking the circuit. It can be readily understood that a fuse made of a high-melting metal imparts more thermal damage to an adjacent part than a fuse made of a low-melting metal. That is, when a fuse made of a high-melting metal is connected in a circuit using an insulated wire for conducting current, the circuit-breaking melt-down area of the fuse generates more heat than the remaining area even under a normal current-conducting condition, and the portions of the insulation covering (whose typical material is polyvinyl chloride) of the wire adjacent to the fuse are heated by the heat transmitted from the circuit-breaking melt-down area of the fuse, resulting in a promoted degradation of those portions of the insulation covering. Further, it is the recent tendency that automobiles of front-engine front-drive design are gaining popularity, and the output of engine becomes higher and higher. Because of the above tendency, the environment of the engine room becomes increasingly severe, and the environmental temperature is rising more and more. Therefore, it has been demanded to suppress the undesirable temperature rise of heat-generating parts even by an amount of 1.degree. C. at the least.
As means for satisfying the demand for minimizing the quantity of generated heat as well as the demand for ensuring a long useful service life, there is a proposal according to which a metal having a relatively low melting point, for example, silver or aluminum, is used to make a fuse. However, a fuse made of silver has the problem of high cost. Also, when aluminum is used to make a fuse, aluminum forms a tight film of its oxide, and a bridge of alumina may remain in a non-melted state even when the fuse is heated due to flow of an overcurrent until finally its melting point is reached. Further, because aluminum is easily corroded even when its oxide film may not be formed, electrolytic corrosion tends to occur between the fuse and a connection terminal or a wire to which the fuse is connected. Therefore, silver and aluminum are not suitable as the material for making fuses, and those made of silver and aluminum are now scarcely put into practical use.